The present invention relates to connecting members such as those used in optical connectors which are located between optical fiber transmission lines for switching, connecting or disconnecting the transmission lines and those directly fitted to ferrules, as well as to optical fiber connecting structures.
Presently, detachable optical connectors are used for connecting optical fiber cables and optical fiber cords intended for short-distance applications such as in-building wiring and wiring to equipment. Optical fibers are so thin and flexible that they are usually secured using ferrules. An optical connector is constructed of a plug incorporating a ferrule and an adaptor incorporating a sleeve.
FIG. 6 shows a cross section of an optical connector employing a conventional crimp ring. As shown in FIG. 6, a ferrule 101 in which an optical fiber 201 is inserted and fixed has a flange 102, and a stopper 104 is fitted behind a rear end portion of the ferrule 101 in the back of the flange 102 with a compression spring 103, which is mounted on the outer periphery of the rear end portion of the ferrule 101, placed in between. This means that the ferrule 101 is fitted in such a way that it can move relative to the stopper 104 in its axial direction with the aid of the compression spring 103. A coated optical fiber 202 is inserted into the compression spring 103 and the stopper 104 with a tensile strength member 204 of an optical fiber cable 203 located at the rear end of the stopper 104, and the tensile strength member 204 is secured around the outer periphery of a rear end portion of the stopper 104 by crimping a crimp ring 105.
The crimp ring 105 is formed of a first cylindrical portion 111 whose inside diameter fits the outer periphery of the rear end portion of the stopper 104, a second cylindrical portion 112 which fits on the outer periphery of the optical fiber cable 203, and a connecting part 113 interconnecting the first and second cylindrical portion 111, 113, as shown in FIG. 7. The first cylindrical portion 111 is crimped to secure the tensile strength member 204 between an inside curved surface 111a of the first cylindrical portion 111 and the stopper 104 and, then, the second cylindrical portion 112 is crimped to firmly hold the optical fiber cable 203 in position with a compressive effect provided by an inside curved surface 112a of the second cylindrical portion 112.
Since the optical fiber cable 203 is held in position with compressive and frictional forces exerted by the inside curved surface 112a of the second cylindrical portion 112 of the crimp ring 105 as described above, its total securing force varies as a result of changes in the outside diameter of the optical fiber cable 203. It is therefore necessary to design the crimp ring 105 with varying inside diameters of the second cylindrical portion 112 to cater for such changes in the outside diameter of the optical fiber cable 203. In addition, the outside diameter of the second cylindrical portion 112 should be made suitable for diameter rating of a crimping tool to be used. Thus, one problem is that the crimp ring 105 is remarkably expensive, because it is conventionally produced by cutting an aluminum material, for instance. Another problem is that the second cylindrical portion 112 becomes relatively thick as a result of a relationship between the outside diameter of the optical fiber cable 203 and the diameter rating of the crimping tool, thereby requiring a great force in crimping operation.
This invention is intended to solve the foregoing problems in the conventional art; and has as an object the provision of clamp rings which are easy to crimp and manufacture.
Also known in the prior art are structures for terminating optical fiber ends by directly fitting a ferrule to each end of an optical fiber cord. An example of such structures is shown in FIGS. 8A-8B. As shown in FIGS. 8A-8B, after inserting a coated optical fiber into a ferrule, a tensile strength member 204 of an optical fiber cable 203 is placed around the periphery 302 of a rear end portion of the ferrule 301, a ring member 303 is fitted, and both ends of the ring member 303 is fixed in position by an adhesive 304 to thereby secure the individual components in integral form.
Such conventional structure has such a problem that a sufficient tensile strength (about 10 kgf) can not be achieved since the tensile strength member 204 is fixed by using the adhesive 304.
Another problem of this conventional structure is that it increases the chance of fiber breakage. This is because the adhesive 304 is sucked into the tensile strength member 204 and that portion of the tensile strength member 204 where the adhesive 304 has been sucked looses flexibility.
The structure has yet another problem in that it requires a good deal of man hours for assembly work, because the adhesive 304 needs a long time to cure.
The invention aims at solving these problems. Accordingly, it is also an object of the invention to provide a structure which makes it possible to directly secure an optical fiber cable to a ferrule without using an adhesive, as well as an optical fiber terminating structure which provides a sufficient tensile strength without causing a loss of optical fiber cable flexibility, yet permitting ease of production.
In a first mode of carrying out the invention, a connecting member is provided for connecting a tensile strength member to the periphery of a rear end portion of a ferrule retaining part for holding a ferrule in which an optical fiber of an optical fiber cable incorporating a coated optical fiber and the tensile strength member surrounding the coated optical fiber is inserted and fixed with an outer covering of the optical fiber cable stripped off from its terminal portion. The connecting member comprises a first annular portion which fits on the periphery of the rear end portion of the ferrule retaining part, a second annular portion located in the back of the first annular portion, the second annular portion having a smaller diameter than the first annular portion, a step portion interconnecting the second annular portion and the first annular portion, and a clamping portion extending inward from the rear end of the second annular portion in such a way that the extreme end of the clamping portion secures the periphery of the optical fiber cable. The first annular portion, the second annular portion, the step portion and the clamping portion are formed as a single piece of metal having approximately the same wall thickness.
The connecting member of the invention preferably comprises a clamping ring and is formed by press-forming operation, for example.
In a second mode of carrying out the invention, an optical fiber connecting structure for connecting a tensile strength member to the periphery of a rear end portion of a ferrule, in which an optical fiber of an optical fiber cable incorporating a coated optical fiber and the tensile strength member surrounding the coated optical fiber is inserted and fixed with an outer covering of the optical fiber cable stripped off from its terminal portion, is characterized in that the tensile strength member is connected to the periphery of a rear end portion of the ferrule by crimping a connecting member with its one end fitted on the periphery of the rear end portion of the ferrule and the other end fitted on the periphery of the optical fiber cable.
In this structure, the connecting member preferably comprises a first annular portion which fits on the periphery of the rear end portion of the ferrule, a second annular portion located in the back of the first annular portion, the second annular portion having a smaller diameter than the first annular portion, a step portion interconnecting the second annular portion and the first annular portion, and a clamping portion extending inward from the rear end of the second annular portion in such a way that the extreme end of the clamping portion secures the periphery of the optical fiber cable. The first annular portion, the second annular portion, the step portion and the clamping portion are formed as a single piece of metal having approximately the same wall thickness.
Preferably, the periphery of the rear end portion of the ferrule is formed into a surface having pits and protrusions.